The present invention relates to an electrodeposited copper foil and a process for continuously producing the electrodeposited copper foil, and more particularly to a technique which allows use of a thiourea-added copper sulfate solution.
In the field of electrodeposition and electroforming of copper, it has conventionally been known that electrolysis by-products and impurities remaining in a copper electrolyte greatly affect the physical properties of electrodeposited products obtained through electrolysis. Among the products, electrodeposited copper foil is used for forming circuits to allow the flow of current in printed wiring boards, and therefore, electric resistance of a required level must be provided. Thus, at the production stage of electrodeposited copper foil, it is desirable to remove undesired impurities and contamination by undesired matter to the greatest possible extent. Generally, such undesirable electrolysis by-products and impurities remaining in a copper electrolyte are removed by a variety of methods; e.g., use of filter cloth, activated carbon, ion-exchange resin, or a similar material.
Among additives for a copper electrolyte, thiourea is known to be a compound capable of imparting remarkably high hardness to electrodeposited copper. Accordingly, there have been investigated a variety of methods for mass-producing electrodeposited copper from an electrolyte to which thiourea alone is added.
However, thiourea incorporated into a copper electrolyte forms, through oxidation such as electrode oxidation or oxidation by oxygen gas, FD (formamidine disulfide), derivatives thereof, thiosulfuric acid, polythionic acid (H2SnO6), and other decomposition products derived from thiourea.
These thiourea-decomposed products are difficult to remove completely through a general filtration method employing filter cloth, activated carbon, ion-exchange resin, or a similar material. In order to prevent formation of thiourea-decomposed products, a compound other than thiourea is used in combination with thiourea, and this has heretofore been the only way which allows the use of thiourea. Thus, it has never been possible to mass-produce electrodeposited copper through use of thiourea as a single additive.